Opus Anglicanum (meaning 'work of the English') are usually, but not always, religious embroideries. They are made of pure silk, pure gold threads and employ, amongst other stitches, a method called underside couching which means that the whole piece is worked from the reverse.
When you consider the instruments the male embroiderers would have had at their disposal - iron needle anyone? - it is astonishing that they managed to produce such detailed and impressive work.
Most of the pieces were worked to commission in the London area and at one stage it was the centre of embroidery for the medieval world. Sadly all that came to an end when the Black Death plague in 1346 wiped out nearly all of the embroiderers working in London at that time. Sadly the skills were lost and the London master craftsmen never regained their previous prowess.
The V&A have 100 of these masterpieces on display until February 2017, although you will need to book and pay for a ticket to see them. More details from https://www.vam.ac.uk/exhibitions/opus-anglicanum-masterpieces-of-english-medieval-embroidery